Synthetic linear polyester filaments, yarns and cords are known to show improved strength under hydrolytic conditions or elevated temperatures when, either by the use of certain additives to the polymer, or chemical coatings on the polyester fiber, the carboxyl end group concentration is lowered.
One of the commonly employed methods of lowering the carboxyl level is by applying to the polyester fiber a surface coating which consists essentially of a carbodiimide or polycarbodiimide. United States patents relating to the use of such carbodiimides or polycarbodiimides include U.S. Pat. No. 3,193,523, U.S. Pat. No. 3,193,524, and U.S. patent application Ser. No. 302,710 filed Nov. 1, 1972.